U.S. patent number 7,012,754 [Application Number 10/926,360] was granted by the patent office on 2006-03-14 for apparatus and method for manufacturing tilted microlenses.
This patent grant is currently assigned to Micron Technology, Inc.. Invention is credited to Ulrich C. Boettiger, Jin Li.
United States Patent |
7,012,754 |
Boettiger , et al. |
March 14, 2006 |
Apparatus and method for manufacturing tilted microlenses
Abstract
Asymmetrical structures and methods are used to adjust the
orientation of a microlens for a pixel array. The asymmetrical
structures affect volume and surface force parameters during
microlens formation. Exemplary microlens structures include an
asymmetrical microlens frame, base, material or a combination
thereof to affect the focal characteristics of the microlens. The
asymmetrical frame alters the microlens flow resulting from the
heating of the microlens during fabrication such that orientation
of the microlens relative to an axis of the imager can be
controlled.
Inventors: |
Boettiger; Ulrich C. (Boise,
ID), Li; Jin (Boise, ID) |
Assignee: |
Micron Technology, Inc. (Boise,
ID)
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Family
ID: |
36652961 |
Appl.
No.: |
10/926,360 |
Filed: |
August 26, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050270653 A1 |
Dec 8, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10857948 |
Jun 2, 2004 |
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Current U.S.
Class: |
359/626; 348/340;
359/620 |
Current CPC
Class: |
B29D
11/00278 (20130101); B29D 11/00365 (20130101); G02B
3/0018 (20130101); G02B 3/0043 (20130101); H01L
27/14627 (20130101); H01L 27/14685 (20130101); G02B
27/10 (20130101); G02B 3/0056 (20130101) |
Current International
Class: |
G02B
27/10 (20060101); H04N 5/225 (20060101) |
Field of
Search: |
;264/1.1,1.32,1.38,2.2,2.6,2.7 ;348/340 ;359/620,626 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Mikael Karlsson, "Micro-Optical Elements in Gallium Arsenide and
Diamond: Fabrication and Applications," Comprehensive Summaries of
Uppsala Dissertations from the Faculty of Science and Technology
811, 2003, pp. 1-73. cited by other.
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Primary Examiner: Spector; David N.
Attorney, Agent or Firm: Dickstein Shapiro Morin &
Oshinsky LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent
application Ser. No. 10/857,948, filed Jun. 2, 2004, the disclosure
of which is incorporated by reference herein.
Claims
What is claimed is:
1. A pixel structure, comprising: a plurality of asymmetrical
containing structures; and lens material located in said
asymmetrical containing structures, said lens material having a
liquid phase and a solid phase, said plurality of asymmetrical
containing structures defining a boundary of said liquid phase lens
material.
2. The imager structure of claim 1, wherein said lens material
after said lens material transitions to the solid-phase forms an
angle different from a zero degree angle with said axis of said
plurality of pixels.
3. The imager structure of claim 2, wherein said lens material
after said lens material transitions to the solid-phase forms an
angle different from a zero degree angle with a horizontal axis of
said plurality of pixels.
4. The pixel structure of claim 1, wherein at least one of said
plurality of asymmetrical containing structures has an asymmetrical
shape.
5. The pixel structure of claim 4, wherein at least one of said
plurality of asymmetrical containing structures has a trapezoidal
shape.
6. The pixel structure of claim 1, wherein said lens material
transitions from said liquid-phase lens material to a convex shaped
solid-phase lens structure after heat processing.
7. The pixel structure of claim 1, wherein said lens material
transitions from said liquid-phase lens material to a concave
shaped solid phase of said lens structure after heat
processing.
8. The pixel structure of claim 1, wherein one or more of said
asymmetrical containing structures are formed from a light
absorbing material or a transparent material.
9. An imager structure, comprising: a plurality of pixels formed in
a substrate; a planar layer formed over said plurality of pixels; a
plurality of asymmetrical openings being formed in said planar
layer, each of said plurality of asymmetrical openings being
respectively formed over one of said plurality of pixels, each of
said plurality of asymmetrical openings comprising a plurality of
sidewalls; and a lens material deposited in each of said plurality
of asymmetrical openings, wherein each of said plurality of
sidewalls contains and defines the outer boundary of said lens
material during a liquid-phase of said lens material which results
in lens material orientation alterations caused in part by heating
of said lens material, and wherein said lens material orientation
after heating is dependent on one or more dimensions of said
plurality of sidewalls during a liquid-phase to solid-phase change
of microlens material in said plurality of asymmetrical
openings.
10. The imager structure of claim 9, wherein at least one of said
plurality of asymmetrical openings has a trapezoidal
configuration.
11. The imager structure of claim 9, wherein said lens material
after heating is tilted relative to a horizontal axis of said
plurality of pixels.
12. The imager structure of claim 9, wherein the focal point of
said lens material after heating is shifted relative to a vertical
axis of said plurality of pixels.
13. A lens structure, comprising: a plurality of asymmetrical
frames provided within a planar layer, said plurality of
asymmetrical frames comprising a plurality of sidewalls, each
asymmetrical frame defining an asymmetrical frame space, wherein
each of said plurality of asymmetrical frames is adapted to have a
liquid-phase lens material disposed therewithin, said liquid-phase
lens material having a surface tension force, said liquid-phase
lens material and said sidewalls having an adhesive force
therebetween, whereby a third surface of said liquid-phase lens
material assumes a topography related to at least said surface
tension force, said adhesive force and a geometric shape of a
perimeter defined by said sidewalls, wherein said liquid-phase lens
material transitions to a solid-phase lens structure after heat
processing, said solid-phase lens structure having a tilted
orientation relative to a horizontal axis of said planar layer.
14. The lens structure of claim 13, wherein at least one of said
plurality of asymmetrical frames comprises a trapezoidal
cavity.
15. The lens structure of claim 13, wherein said liquid-phase lens
material transitions to a convex shaped solid-phase lens structure
after heat processing.
16. The lens structure of claim 15, wherein said convex shaped
solid-phase lens structure is a semi-spherical microlens.
17. The lens structure of claim 16, wherein a diameter of said
semi-spherical microlens forms an angle other than a zero degree
angle with said horizontal axis of said planar layer.
18. The lens structure of claim 13, wherein said liquid-phase lens
material transitions to a concave shaped solid-phase lens structure
after heat processing.
19. The lens structure of claim 18, wherein said concave shaped
solid-phase lens structure is a semi-spherical microlens.
20. The lens structure of claim 19, wherein a diameter of said
semi-spherical microlens forms an angle other than a zero degree
angle with said horizontal axis of said planar layer.
21. An image processing system, comprising: a semiconductor
substrate; a plurality of image pixels formed in said semiconductor
substrate; and a planar layer formed over said image pixels, said
planar layer having a plurality of lens structures comprising a
plurality of asymmetrical frames; wherein each of said plurality of
asymmetrical frames is adapted to have a liquid-phase lens material
disposed therewithin, said liquid-phase lens material having a
surface tension force, said liquid-phase lens material and said
sidewalls having an adhesive force therebetween, whereby a third
surface of said liquid-phase lens material assumes a topography
related to at least said surface tension force, said adhesive force
and a geometric shape of a perimeter defined by said sidewalls,
wherein said liquid-phase lens material transitions to a
solid-phase lens structure after heat processing, said solid-phase
lens structure having a tilted orientation relative to a horizontal
axis of said planar layer.
22. The image processing system of claim 21, wherein at least one
of said plurality of asymmetrical frames comprises a trapezoidal
cavity.
23. The image processing system of claim 21, wherein said
liquid-phase lens material transitions to a convex shaped
solid-phase lens structure after heat processing.
24. The image processing system of claim 21, wherein said
liquid-phase lens material transitions to a concave shaped
solid-phase lens structure after heat processing.
25. A method of forming an imager structure, comprising: forming a
plurality of pixels in a substrate; depositing a planar layer over
said plurality of pixels; forming a plurality of asymmetrical
containing structures within said planar layer, said plurality of
asymmetrical containing structures comprising a plurality of
asymmetrical sidewalls from said planar layer over said plurality
of pixels; forming a microlens structure in each one of said
plurality of asymmetrical containing structures from lens material
deposited within said asymmetrical containing structures; and
transitioning said lens material to a liquid-phase for a
predetermined time to permit flowing of said lens material, wherein
each of said asymmetrical sidewalls exerts a force on said lens
material during the liquid-phase that alters the orientation of
said lens material relative to an axis of said planar layer after
said lens material transitions to a solid-phase.
26. The method of claim 25, wherein the orientation of said lens
material relative to said axis of said planar layer is dependent on
at least one dimension of said asymmetrical sidewalls.
27. The method of claim 25, where said step of forming a plurality
of asymmetrical containing structures further comprises forming
said plurality of asymmetrical sidewalls.
28. The method of claim 25, wherein said step of forming a
plurality of asymmetrical containing structures further comprises
forming a trapezoidal containing structure.
29. The method of claim 25, wherein said lens material transitions
from said liquid-phase lens material to a convex shaped solid-phase
lens structure after heat processing.
30. The method of claim 25, wherein said lens material transitions
from said liquid-phase lens material to a concave shaped solid
phase said lens structure after heat processing.
31. The method of claim 25, wherein one or more of said
asymmetrical containing structures are formed from a transparent or
light absorbing material.
32. The method of claim 25, wherein said predetermined time for
said step of transitioning said lens material to a liquid phase is
determined based on a time period for said lens material to
transition to a solid-phase before said liquid-phase lens material
achieves equilibrium.
33. A method of forming a lens, comprising: providing a layer of a
first material over a plurality of photosensitive devices; forming
an asymmetrical frame within said layer of a first material, said
asymmetrical frame comprising a plurality of surfaces defining an
opening; disposing a unit of solid-phase lens material within said
asymmetrical frame; liquefying said unit of solid-phase lens
material to form a unit of liquid-phase lens material within said
frame, said unit of liquid-phase lens material having a first
surface tension force therewithin; adhering a first surface of said
liquid-phase lens material to a second surface of said asymmetrical
frame in response to a second adhesive force between said
liquid-phase lens material and said asymmetrical frame; and
solidifying said unit of liquid-phase lens material to form a
solid-phase lens having a tilted orientation relative to a
horizontal axis of said plurality of photosensitive devices.
34. The method of claim 33, wherein said first material of said
layer is similar to said solid-phase lens material.
35. The method of claim 33, wherein said first material of said
layer is different from said solid-phase lens material.
36. The method of claim 33, wherein said step of forming an
asymmetrical frame further comprises setting a lithographic stepper
used in forming said asymmetrical frame with one or more settings
including a focus, dose, partial coherence and numerical aperture
to further control a height, width or length of one or more
portions of said asymmetrical frame.
37. The method of claim 33, wherein said liquefying step comprises
heating said solid-phase lens material to a temperature within the
range of about 150 to 220 degrees Celsius.
38. The method of claim 33, wherein said step of disposing said
unit of solid-phase lens material within said asymmetrical frame
comprises over-filling said asymmetrical frame with lens material
such that said liquid-phase lens material is formed into a convex
lens.
39. The method of claim 33, wherein said step of disposing said
unit of solid-phase lens material within said asymmetrical frame
comprises under-filling said asymmetrical frame with lens material
such that said liquid-phase lens material is formed into a concave
lens.
40. A method of shifting the focal point of a microlens of an
imaging device, comprising: providing a plurality of photosensitive
elements in a substrate; forming a layer of a first material over
said plurality of photosensitive elements; and providing at least
one semi-spherical microlens over at least one of said plurality of
photosensitive elements, wherein a diameter of said semi-spherical
microlens forms an angle different from a zero degree angle with a
horizontal axis of said at least one of said plurality of
photosensitive elements, wherein said step of providing at least
one semi-spherical microlens further comprises: forming a plurality
of asymmetrical cavities within said layer of said first material,
each of said cavities being formed over one of said plurality of
photosensitive elements; forming a plurality of lens material
respectively within each of said plurality of asymmetrical
cavities; heating said plurality of lens materials to transition
said lens material to liquid-phase lens material; and solidifying
said liquid-phase lens material to form a solid-phase lens having a
tilted orientation relative to said horizontal axis of said at
least one of said plurality of photosensitive elements.
41. The method of claim 40, wherein said asymmetrical cavities
include a plurality of asymmetrical sidewalls.
42. The method of claim 40, wherein said material of said first
layer is different from said lens material.
43. The method of claim 40, wherein said material of said first
layer is the same as said lens material.
44. The method of claim 40, wherein at least one of said
photosensitive elements is a photodiode.
45. The method of claim 40, wherein at least one of said
photosensitive elements is a photoconductor.
46. The method of claim 40, wherein at least one of said
photosensitive elements is a phototransitor.
47. The method of claim 40, wherein said photosensitive elements
are part of a CMOS imager.
48. The method of claim 40, wherein said photosensitive elements
are part of a CCD device.
49. The method of claim 40, wherein said photosensitive elements
are part of a photodiode array.
Description
FIELD OF THE INVENTION
The invention relates to fabrication of microlens structures for
image capture or display systems and, more specifically, to
structures and methods of manufacturing of microlens arrays for
solid state imager systems.
BACKGROUND OF THE INVENTION
Solid state imagers, including charge coupled devices (CCD) and
CMOS sensors, have been used in photo imaging applications. A solid
state imager circuit includes a focal plane array of pixel cells,
each one of the cells including either a photogate, photoconductor
or a photodiode having a doped region for accumulating
photo-generated charge. Microlenses are placed over imager pixel
cells. A microlens is used to focus light onto the initial charge
accumulation region. A single microlens may have a polymer coating,
which is patterned into squares or circles provided respectively
over the pixels which are then heated during manufacturing to shape
and cure the microlens.
Use of microlenses significantly improves the photosensitivity of
the imaging device by collecting light from a large light
collecting area and focusing it on a small photosensitive area of
the sensor. The ratio of the overall light collecting area to the
photosensitive area of the sensor is known as the fill factor of
the pixel.
The use of smaller sized microlens arrays is increasingly important
in microlens optics. One reason for increased interest in reducing
the size of microlenses is the increased need to reduce the size of
imager devices and increase imager resolution. However, reductions
in pixel sizes result in a smaller charge accumulation area in
individual pixels in the array. Reduced sizes of pixels result in
smaller accumulated charges which are read out and processed by
signal processing circuits.
As the size of imager arrays and photosensitive regions of pixels
decreases, it becomes increasingly difficult to provide a microlens
capable of focusing incident light rays onto the photosensitive
regions. This problem is due in part to the increased difficulty in
constructing a smaller microlens that has the optimal focal
characteristics for the imager device process and that optimally
adjusts for optical aberrations introduced as the light passes
through the various device layers. Also, it is difficult to correct
possible distortions created by multiple regions above the
photosensitive area, which results in increased crosstalk between
adjacent pixels. "Crosstalk" can result when off-axis light strikes
a microlens at an obtuse angle. The off-axis light passes through
planarization regions and a color filter, misses the intended
photosensitive region, and instead strikes an adjacent
photosensitive region.
Microlens shaping and fabrication through heating and melting
microlens materials becomes increasingly difficult as microlens
structures decrease in size. Previous approaches to control
microlens shaping and fabrication do not provide sufficient control
to ensure optical properties such as focal characteristics or other
parameters needed to provide a desired focal effect for smaller
microlens designs. Consequently, imagers with smaller sized
microlenses have difficulty in achieving high color fidelity and
signal/noise ratios.
BRIEF SUMMARY OF THE INVENTION
Exemplary embodiments of the present invention provide easily
manufactured microlenses which can be used in an imager or display
device. The microlens structures of the invention are formed by
employing asymmetrical frame structures of a first material that
affect volume and surface force parameters during microlens
formation. The asymmetrical frame structures affect the orientation
of at least one microlens of a second material with respect to at
least one axis of the imager or the display device (for example,
the horizontal axis of the imager). The asymmetrical frame
structures alter the microlens flow resulting from the heating of
the microlens during fabrication, such that the microlens
orientation with respect to at least one axis of the imager or the
display device can be controlled. Also provided are methods of
forming a microlens structure by (i) forming an asymmetrical frame
structure over a substrate and (ii) adding a lens material into the
asymmetrical frame structure to form at least one tilted
microlens.
These and other features representative of various embodiments of
the invention will be more readily understood from the following
detailed description of the invention, which is provided in
connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a cross-sectional view of a portion of an
asymmetrical frame for microlens structures constructed in
accordance with an exemplary embodiment of the invention;
FIG. 2 illustrates a top view of the portion of the asymmetrical
frame of FIG. 1;
FIG. 3 illustrates a perspective view of a portion of the
asymmetrical frame of FIG. 1;
FIG. 4 illustrates a perspective view of the portion of the
asymmetrical frame of FIG. 3 with lens material before heating;
FIG. 5 illustrates a perspective view of the portion of the
asymmetrical frame of FIG. 3 with lens material after heating;
FIG. 6 illustrates a cross-sectional view of a portion of a
microlens structure fabricated with the asymmetrical frame of FIG.
1 and with lens material after heating;
FIG. 7 is a partial cross-sectional view of the microlens structure
of FIG. 6;
FIG. 8 is a schematic cross-sectional view of the microlens
structure of FIG. 6;
FIG. 9 illustrates a top view of a portion of an asymmetrical frame
for microlens structures constructed in accordance with another
exemplary embodiment of the invention;
FIG. 10 is a schematic cross-sectional view of a microlens
structure fabricated with the frame of FIG. 9;
FIG. 11 is a perspective view of a portion of an asymmetrical frame
for microlens structures constructed in accordance with another
exemplary embodiment of the invention;
FIG. 12 is a perspective view of another portion of an asymmetrical
frame for microlens structures constructed in accordance with yet
another exemplary embodiment of the invention;
FIG. 13 is a perspective view of a portion of an asymmetrical frame
for microlens structures constructed in accordance with yet another
exemplary embodiment of the invention;
FIG. 14 illustrates a flow chart for a method of manufacturing a
microlens structure in accordance with an exemplary embodiment of
the invention; and
FIG. 15 illustrates an image processing system constructed in
accordance with an exemplary embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
According to the invention, the focal characteristics of a
microlens and its orientation can be adjusted by forming boundary
walls around lens materials to contain and/or limit the outward
flow of lens material. This adjusts microlens flow behavior by, for
example, increasing the height of the microlens or defining a
perimeter of the microlens structures. Boundary walls can be formed
by forming cavities or frame sidewalls with microlens material
deposited therein. Post-flow microlens orientation can also be
adjusted or affected by use of asymmetrical structures formed
within the microlens materials (for example, a microlens base
within a microlens material) that change flow behavior of the
microlens during microlens flow processing (e.g., altering the
orientation of the microlens from a planar layer or a microlens
cavity). In this manner, asymmetrical microlenses can be formed
with asymmetrical structures to provide focal points that are not
directly underneath a microlens as needed for a particular
design.
The orientation and optical properties of a microlens can be
adjusted independently of other microlens design parameters by
using structures, materials and/or fabrication processing
techniques to affect the microlens melting or flow behavior during
fabrication. As used in this application, a positive microlens
includes a microlens with an inner portion extending above a layer
or above a microlens cavity (e.g., a convex microlens) and a
negative microlens includes a microlens that does not extend
outside the microlens cavity or below the layer (e.g., a concave
microlens).
Exemplary embodiments provide an alteration of the balance of
surface and volume related forces, by creating a microlens within
at least one asymmetrical enclosure or frame; this results in an
altered orientation of the microlens (after heating and subsequent
flowing of melted microlens material). A variety of asymmetrical
enclosures or frame shapes can be used to create the microlens with
a desired optical property, focal characteristics and/or desired
orientation. For example, by providing an asymmetrical frame
structure, the orientation of the reflowed microlens can be
controlled so that the focal spot of the microlens can be shifted
to the center of the light sensitive area of interest.
As described below, microlens material is heated and flowed during
microlens fabrication by melting the microlens material, resulting
in a flow of the deposited material into a desired asymmetrical
shape. For example, a trapezoidal block of microlens material
formed on a planar surface will assume a tilted semi-spherical
shaped microlens after being heated to 170.degree. C. to
180.degree. C. (depending on resist) due to the melting and flowing
of the microlens material. The term "flow", "flowing" or
"reflowing" refers to a change in shape of a material which is
heated and melts thereby producing a material flow or shape
alteration in the material caused by heating or other similar
process. "Flow" is an initial melting and "reflow" is a subsequent
melting of an asymmetrical frame or microlens material that has
been previously flowed.
Referring now to the drawings, where like elements are designated
by like reference numerals, FIG. 1 illustrates a cross-sectional
view of an asymmetrical microlens frame 10 having adjacent openings
5, which will be subsequently filled with lens material to form a
tilted semi-spherical microlens. The asymmetrical frame 10 may be
employed, for example, in a two-way shared pixel design application
wherein two photosensitive elements (for example, two photodiodes)
are grouped together to help pixel shrinkage and allow more packing
of logic circuits. The asymmetrical frame 10 is formed over a
substrate 3 having photosensor devices 4 therein. Asymmetrical
frame regions A and B are each defined by frame openings 5, which
in turn are formed within a planar layer 1 of a first material
formed over the substrate 3 using methods well known in the art,
such as etching or patterning. Frame openings 5 are defined by
frame opening sidewalls 2, 2a and a flat floor portion 7. Each
opening sidewall 2 of each frame opening 5 has a first width "D,"
which is a greater than a second width "w" characterizing the
opening sidewalls 2a. For the purposes of the present invention,
the terms "asymmetrical structure"; "asymmetrical containing
structure"; "asymmetrical openings"; "asymmetrical frames"; and
"asymmetrical cavities" are defined as particular spatial
structures having geometrical configurations which enable the
formation of a tilted microlens structure by methods of the present
invention.
FIG. 2 illustrates a top view of the asymmetrical frame regions A,
B of the asymmetrical frame 10 of FIG. 1, illustrating frame
openings 5 formed within the planar layer 1 of the first material.
As shown in FIG. 2, frame openings 5 of regions A, B have an
asymmetrical trapezoidal shape. An elevational perspective view of
the asymmetrical frame region A of FIG. 2 is illustrated in FIG. 3.
Sidewall 2 of the frame opening 5 has a first height "H" of about
0.5 microns that slopes down laterally on both sides towards the
sidewall 2a, which has a second height "h" of about 0.2 microns.
The width of the sidewalls may also vary. For example, as shown in
FIG. 3, sidewall 2 has a first width "D" of about 0.25 to 1.0
microns, whereas the width of sidewalls 2b decreases from the first
width "D" to a second width "w" of about 0.2 to 0.5 microns. In
this manner, the four sidewalls 2, 2b, 2a, 2b and floor portion 7
define a spatial trapezoidal region within the asymmetrical frame
opening 5 of the frame region A. As discussed below, microlens
material is deposited into the asymmetrical frame opening 5 and
then heated to flow or melt into a desired tilted microlens.
Reference is now made to FIG. 4. A deposited microlens layer 17 is
formed within the frame opening 5 of the frame region A. The
microlens layer 17 is formed of a second material which may be
similar to, or different from, the first material of the frame
region A. Thus, the microlens layer 17 may be formed by coating the
substrate 3 with positive resist 17 (if convex tilted microlenses
are desired) to form an asymmetrical structure following the shape
of the asymmetrical frame opening 5. FIG. 5 illustrates the
structure of FIG. 4 after reflow of the deposited microlens layer
17, wherein the microlens layer 17 melts and flows such that it
assumes a tilted shape.
As illustrated in FIGS. 6 and 7, the orientation of resulting
reflowed semi-spherical microlens 100 is tilted, in that axis 42
which is tangential to the highest and lowest points of the
sidewalls 2, 2a forms an angle ".alpha." different than a zero
degree angle with horizontal axis 52 of the substrate 3. In this
manner, tilted microlenses can be formed with an orientation
independent of the size of the microlens. For example, the tilted
microlenses may have an orientation that allows their focal spot to
shift to a target location (a photosensitive element such as a
photodiode, for example). By providing the two tilted microlenses
as part of a two-way shared pixel layout, it is possible to shift
the focal point of each of the microlenses to the targeted device
(i.e., the photosensor device 4). In addition, the two targeted
devices can be placed closer together, allowing more pixel area for
logic circuitry. Further, the two microlenses may be formed over
only one photosensitive element (for example, over the left side
photosensor device 4 of FIG. 8). Of course, in such case, the right
side microlens 100 must be tilted more relative to the horizontal
axis of the photodiode 4 (and also relative to the left side
microlens 100). Moreover, by controlling the degree of tilt
relative to a photosensitive element of the imager, more freedom in
the design of the photosensitive element is permitted, since the
photosensitive design is not limited to the pixel center, and the
focal point of the tilted microlens can be shifted to where the
photosensitive element is placed within the pixel.
FIG. 8 schematically illustrates two tilted convex shaped
microlenses 100 formed as described above and as part of a two-way
shared pixel 101 comprising two photodiodes 4. The orientation of
the tilted microlens, such as the tilted microlens 100 of FIG. 8,
as well as the dimensions, shape, volume of lens frame opening
(e.g., opening 5), focal length and other focal characteristics in
the embodiments that use asymmetrical frames are determined by one
or more microlens and imager design parameters including: (1) the
distance, width or size of the photosensor 4 underneath the
asymmetrical frame opening 5 where the microlens focuses light; (2)
the viscosity of the microlens material used to form the
microlenses during heating; (3) the sidewall dimensions of the
asymmetrical structures or frame opening (e.g., the height and
width of the sidewall 2) that affect microlens formation; (4) the
alterations in flow behavior of the microlens material resulting
from microlens structures affecting microlens material flow
behavior during heating; (5) the effects of pre-heating or pre-flow
treatment of frame or microlens materials; (6) the desired
approximate orientation of the microlens structure after heating of
the microlens material is completed; and (7) the effects of a base
layer within microlens material that alters flow properties of the
microlens material.
Pre-flow treatment of asymmetrical frame or microlens materials can
include UV exposure which cross-links the material resulting in
less flow and better shape retention of "as printed" microlens
shapes. Reflow properties of microlens materials, asymmetrical
frames structures receiving microlens material and microlens base
layers embedded in microlens material are determined by initial
polymer material properties of microlens material, frame opening
structures and/or microlens base layers, among others. Also,
temperature profile over time for reflow, pre-flow treatment of
material and pre-bake of microlens, frame or layer materials (or a
combination thereof) at temperatures below glass transition
temperatures for a specified time that tends to harden a material
so treated. Ultraviolet (UV) or light exposure before reflow can
affect the viscosity of microlens material or frame material during
flow reflow processing. Pre-baking results in less reflow and
better "as printed" shape retention of microlens material, material
which asymmetrical frame openings are formed into (e.g., planar
layer 1) or both.
Asymmetrical frame opening 5 can be designed with a variety of
sidewalls having different profiles that affect the microlens
material deposited and flowed within the frame opening 5 and
ultimately their orientation within various pixel designs. For
example, FIG. 9 illustrates another embodiment of the present
invention, according to which four asymmetrical frame openings 15
are formed as part of a four-way shared pixel design 201. Each of
the asymmetrical frame openings 15 may be similar to, or different
from, the asymmetrical frame opening 5 to allow a tilted microlens,
such as convex shaped tilted microlens 200 of FIG. 10, to be formed
after reflow. In this manner, asymmetrical microlenses can be
formed with asymmetrical frame openings to provide focal points
that are not directly underneath a microlens if needed for a
particular design. By providing the four tilted microlenses as part
of a four-way shared pixel layout, it is possible to shift the
focal point of each of the microlenses to the targeted device
(i.e., the photosensor device 4). In addition, the targeted devices
can be placed closer together, allowing more pixel area for logic
circuitry. Further, the four microlenses 200 may be formed over
only one photosensitive element (for example, over the left side
photosensor device 4 of FIG. 10). Of course, in such case, some of
the microlenses 200 must be tilted more relative to the horizontal
axis of the photodiode 4 and also relative to the other
microlenses. Moreover, by controlling the degree of tilt relative
to a photosensitive element of the imager, more freedom in the
design of the photosensitive element is permitted, since the
photosensitive design is not limited to the pixel center, and the
focal point of the tilted microlenses can be shifted to where the
photosensitive element is placed within the pixel.
FIGS. 11 13 illustrate exemplary embodiments of additional
asymmetrical frame openings 25, 35, 45 formed of sidewalls having
various thicknesses and widths, which are selected so that the flow
or reflow behavior of a microlens material during heating results
in different microlens orientations after flowing or reflowing. For
example, FIG. 11 illustrates a perspective view of asymmetrical
frame opening 25 having sidewalls 22, 22a and 22b with similar
heights relative to the floor portion of the planar layer 1.
However, the width of the sidewalls differs. As shown in FIG. 11,
sidewall 22 has a constant first width "D," sidewall 22a has a
constant second width "w" which is different from the first width
"D" and sidewalls 22b each have a varying width "w.sub.2" which
decreases from the first width "D" to the second width "w." The
asymmetrical frame opening 25 may be part of any desired layout,
for example, of a two-way shared diode layout.
FIG. 12 illustrates asymmetrical frame opening 35 defined by
sidewalls 32, 32a and 32b having varying heights and varying
widths. As shown in FIG. 12, sidewall 32 and one of sidewalls 32b
have a constant height "H" and a constant width "D," whereas the
other sidewall 32b and sidewall 32a have a varying height (from "H"
to "h") and a varying width (from "D" to "w"). FIG. 13 illustrates
another exemplary embodiment of the present invention, according to
which asymmetrical frame opening 45 is defined by sidewalls 42, 42a
and 42b. Sidewalls 42a and 42 have constant heights "H" and "h,"
respectively, and constant width "D." However, sidewalls 42b have a
varying height from "H" to "h" but constant width "D." The
asymmetrical frame openings 35, 45 may be part of any desired
layout, for example, of a two-way or four-way shared diode layout.
Other asymmetrical shapes and configurations can be used to adjust
flow behavior of lens material including other asymmetrically
shaped sidewalls or containing structures of lens material.
A frame sidewall can be further designed based on a
three-dimensional model of the effects of varying multiple
microlens structure design parameters including frame opening
(e.g., opening 5) or volume affecting structures to design an
asymmetrical frame opening volume, floor and sidewalls that
suitably alter microlens material flow behavior towards a desired
microlens shape. A three-dimensional profile of asymmetrical frame
opening 5 can be designed based on design layout as well as
adjusting photolithography tool characteristics. For example,
three-dimensional profiles can be obtained as the result of a
limited resolution of a photolithography tool and process used to
print or form the frame openings 5. A sloped wall of asymmetrical
frame opening 5 can be obtained by exceeding photolithography
resolution limits so a mask produces less than a well defined print
image during photolithography. A three-dimensional profile of a
frame opening 5 can be designed to alter multiple aspects of a
microlens shape or optical properties such as focal
characteristics, for example focal length and focal point.
Three-dimensional modeling and designs for microlenses can be
produced by use of commercial and proprietary optical property
modeling tools that simulate optical characteristics of
microlenses, such as Lighttools.RTM.. Other tools are available to
provide modeling of flow/reflow behavior of microlenses during
heating processing. Together, the tools can be used to predict how
different three-dimensional designs of frame openings, as well as
other structures affecting microlens material flow, alter microlens
shapes and optical properties.
An exemplary process for manufacturing exemplary embodiments of the
invention, such as the ones shown in FIGS. 1 13, is shown in FIG.
14. At processing segment S21, a plurality of pixels including at
least one photosensitive element are formed in a substrate. At
processing segment S22, a transparent resist layer of a first
material is formed over the plurality of pixels. A plurality of
asymmetrical containing structures are formed from the transparent
resist layer of the first material, by exposing and developing the
transparent resist layer according to a first mask design layout of
each microlens orientation group. The asymmetrical containing
structures comprise a plurality of sidewalls formed over the
plurality of pixels, the plurality of containing structures
defining an outer boundary of the microlens structure. Optionally,
at processing segment S23, the asymmetrical containing structures
are thermally reflowed to harden the transparent resist layer of
the first material. At processing segment S24, a transparent resist
layer of a second material is formed over the asymmetrical
containing structures by employing a second mask design layout so
that, at processing segment S25, tilted microlenses are formed by
thermally reflowing the combined resist layers. In this manner,
lens material comprising the second material is formed within each
of the asymmetrical containing structures, the sidewalls of which
exerting a force on the lens material during a liquid-phase of the
lens material that alters the lens orientation during the
liquid-phase of the lens material. Additional processing steps to
complete the formation of the imager comprising at least one tilted
microlens, such as the tilted microlens 100 of FIG. 8, may be
conducted at processing step S26.
A system 69 comprising an imager 73 with a pixel array 77
comprising at least one tilted microlens structure fabricated in
accordance with any of the embodiments of the invention is
illustrated in FIG. 15. The system 69 is coupled with a sample and
hold circuit 83. An analog to digital (A/D) 85 receives signals
from the sample and hold circuit 83 and outputs digital signals to
an image processor 85. The imager 71 then outputs image data to a
data bus 75. A central processing unit coupled to the data bus 75
receives image data.
Although the embodiments illustrated above have been described with
reference to particular examples of asymmetrical frame structures
for fabrication of tilted microlenses, it must be understood that a
variety of sidewall shapes and structure configurations may be
employed in accordance with the invention. In addition, it is also
possible to form a microlens layer relative to a frame layer to
allow additional influences, such as a non-equilibrium based design
parameter, on a final orientation of the completed microlens. For
example, a tilted microlens can be formed based on non-equilibrium
flow conditions such as heating a lens material such that it begins
a phase change to a liquid and begins flowing, then allow the lens
material to solidify during the flow process before it reaches its
final equilibrium shape as defined by surface and volume forces.
Such non-equilibrium based design influences provide additional
means for defining a final orientation of a microlens.
Frame material can be transparent with a certain refractive index
to help guide incident light onto a photosensor, for example a
photodiode. Frame material can also be absorbing to act as a black
matrix outer containment layer. Transparent material or light
absorbing material embodiments would aid in reducing crosstalk. In
addition, although the invention has been described with reference
to the formation of a convex shaped tilted microlens, the invention
also applies to the formation of concave shaped tilted
microlenses.
While exemplary embodiments of the invention have been described
and illustrated, it should be apparent that many changes and
modifications can be made without departing from the spirit or
scope of the invention. Accordingly, the invention is not limited
by the description above, but is only limited by the scope of the
appended claims.
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